I’ve thought about this too, if dark matter is affected by gravity but not other forces, wouldn’t that mean if we were able to drill to the center of any substantial gravity well (say the moon rather than the earth), would we be able to detect a big clump of dark matter? Does that mean there’s a bunch of dark matter at the center of all the stars?
If we assume dark matter is a particle that only reacts via gravity, well, I'm not so sure you will.
The question here is how your particle that only reacts via gravity loses energy. First it will have whatever momentum it has from the galaxy relative to the speed of our solar system and planet. Then as it falls into our gravity well it picks up even more speed. When it reaches the center of our planet it's hauling ass with no brakes so no reason to stop, so it goes screeching out the other side. Even if you somehow had a dark matter particle at 0 relative motion to your gravity well before it fell in, how long is it going to take to bleed off its gravitational energy?
I find the description of dark matter confusing. I hear statements like "dark matter has negligible interaction with ordinary matter". So, how do scientists know that dark matter exists?
"The answer is that our galaxies spin too fast for the visible matter alone to hold them together. Dark matter, which makes up 27% of the universe, provides the additional gravitational force needed. In contrast, visible matter only makes up 5%."
However, I'm left wondering whether dark matter does or doesn't interact significantly with visible matter.
I've always wondered that. like wouldn't uranium, gold, and other heavier elements be more dominant the closer you get to the center of the core? but they are pretty confident on this iron thing
1. There's far less of such heavier elements in the Universe as a whole. Iron is the end-result of all stellar fusion. Heavier elements form only in massive astronomical events --- novas of huge stars, or in neutron-star collisions, black-hole formation, and the like. So most of the heavy stuff is iron or adjacent elements.
2. Most of the heavy stuff most likely is in the inner core, though what the fluid-dynamics of this during and following planet formation are ... is tenuously known at best. Most likely arrived at through modeling and some very limited remote sensing (seismic, neutrino flux), density estimates, and gravimetr (noting differences in Earth's gravitational field).
3. Most of the crustal prevalence of trans-iron elements in the Earth's crust is likely a result of late bombardment. The Giant Impact Hypothesis thought to have formed the Moon may variously account for greater prevalence (more material deposited to the crust from deeper within the Earth) or less (re-liqufication of the crust and mantle leading to more heavy material sinking to greater depths).
4. From an earlier HN thread, @perihelions states that evidence suggests that there's not a sufficient concentration of fissible elements to support sustained nuclear chain reactions within Earth's core, though radioactive decay does contribute about half the source of geothermal energy within the Earth (the other half being latent heat from gravitational energy of formation). See: <https://news.ycombinator.com/item?id=31281424>
Whichever way, generally, yes, heavier elements increase in prevalence with depth.
From Wikipedia:
The core is thus believed to largely be composed of iron (80%), along with nickel and one or more light elements, whereas other dense elements, such as lead and uranium, either are too rare to be significant or tend to bind to lighter elements and thus remain in the crust (see felsic materials).
